Selective spatial mode attenuator using phase-intensity-phase modulation toward mode division multiplexing transmission

In a mode division multiplexing (MDM) transmission using few-mode fibers (FMFs), differential modal gain (DMG) in amplifiers and mode dependent loss (MDL) cause deterioration in signal quality. Therefore, the techniques which selectively attenuate multiplexed modes and equalize optical powers among mode channels are required for a long-haul MDM transmission. In this paper, we propose a selective mode attenuator using phase-intensity-phase (PIP) modulation. The PIP modulation, consisting of a cascade of two phase and one intensity modulation masks connected by optical Fourier transforms (OFTs), makes it possible to selectively attenuate multiplexed mode channels with high accuracy. In the proposed method, the intensity distributions of spatial modes are converted by the phase modulation and OFT, before doing attenuation by intensity mask located between the two phase masks. Due to the action of a pair of phase masks having phase conjugate relations, accurately selective mode attenuation can be performed even if the number of modes is increased over three. To confirm the basic operation of the proposed method, we perform a numerical simulation for power equalization among six spatial modes (LP01, LP11a, LP11b, LP21a, LP21b, and LP02) having different powers. The phase and intensity masks are designed by using simulated annealing. Moreover, we also evaluate modal crosstalk (MXT) characteristics and the wavelength dependence of the equalization in C-band. The results show that the optical powers of all modes are successfully equalized for any wavelengths and the MXT smaller than -25 dB were achieved between all modes.